Fine Structure Regulation Of Ni-Based Catalysts And Their Performance Towards Selective Hydrogenation Reactions

Selective hydrogenation provides a vital approach to efficiently synthesize and upgrade chemicals in the modern chemical industry.The design and preparation of high-efficient heterogeneous catalysts to achieve effective control over hydrogenation selectivity has always been the focus of attention.In recent years,researchers have proposed a series of catalyst design and modification strategies（such as adjusting the size and shape of metal particles,regulating metal-support interactions,and constructing bimetallic systems,etc.）,from which considerable progress have been made in elevating the activity and selectivity of hydrogenation reactions.However,the following challenges still remain:the traditional preparation methods are difficult to control a high dispersion and fine structure uniformity of metal catalysts due to the disadvantages of high toxicity of precursors,high preparation cost and complex process.The rational design of high-performance catalysts is limited by the lack of understanding of the fine structure of catalysts and the structure-property correlation at the atomic level,especially the challenges of monitoring the reaction process through in-situ/dynamic reaction conditions,identifying the intrinsic active sites and revealing the reaction mechanism.In this dissertation,based on the rich tunability of the structure and composition of layered double hydroxides（LDHs）as well as the unique topological transformation characteristics,three kinds of highly-efficient catalysts for selective hydrogenation reactions（hydrodeoxidation of furfural,hydrogenation of nitroaromatic compounds and reductive amination of aldehydes）were obtained.The structure-property relationship was investigated at atomic level through in-situ characterizations and theoretical calculations,and the intrinsic active sites and reaction mechanism of catalyst were revealed.This dissertation provides a feasible approach for fine structure regulation and the enhancement towards catalytic performance of highly-efficient selective hydrogenation catalysts.The specific research contents and results are as follows:1.NiMo intermetallic compound with atomically-ordered active sites towards hydrodeoxygenation of furfuralNiMo alloy and NiMo intermetallic compound（IMC）catalysts were prepared by tuning the in situ co-reduction process of Mo-decorated NiAl-LDHs.NiMo IMC displays excellent catalytic performance for furfural hydrodeoxygenation（HDO）to 2-methylfuran（MF）（yield:99%）at a rather low hydrogen pressure（0.1 MPa）.This is significantly superior to NiMo alloy（37%）,monometallic Ni（9%）and other Ni-based catalysts previously reported,and even comparable to noble metal catalysts.The results of HAADF-STEM,FT-IR of CO adsorption and in situ XAFS show that the Ni/Mo sites in NiMo IMC are distributed in highly-ordered alternating sequence,in contrast to the random atomic distribution in NiMo alloy.In situ FT-IR spectra and DFT calculations demonstrate that the atomically-ordered sites in NiMo IMC determine the tilted adsorption configuration of furfural molecules,which inhibits the direct hydrogenation of C=C bond;while the Ni-Mo bimetallic active center boosts the activation and cleavage of the C-OH group of the reaction intermediate（furfuryl alcohol）,resulting in its exclusive selectivity toward 2-MF.This NiMo IMC catalyst shows the advantage of high selectivity of target product at a relatively low pressure,which can be potentially used in the field of biomass catalytic upgrading.2.Highly-efficient Ru Nisingle-atom alloy catalysts towards chemoselective hydrogenation of nitroarenesA Ru Nisingle atom alloy catalyst supported on Al₂O₃ substrate was prepared based on a two-step synthesis method including a structural topological transformation of LDHs followed by a galvanic replacement treatment.The Ru NiSAA exhibits extraordinary catalytic performance for chemoselective hydrogenation of 4-nitrostyrene to 4-aminostyrene（yield>99%）;and the turnover frequency（TOF）value reaches up to～4300 h^-1,which stands at the highest level among heterogeneous catalysts ever reported under analogous reaction conditions.Electron microscopic and spectroscopic characterizations show that isolated Ru atoms are dispersed on Ninanoparticles surface to form a stable Ru–Nicoordination,which results in the negative Ru sites(Ru^δ–)due to electron transfer from sub-surface Nito Ru.In situ FT-IR,XAFS investigations and density functional theory（DFT）calculations confirm that Ru–Niinterfacial sites as intrinsic active centers facilitate the activation adsorption of nitro-group via both Ru-O and Ni-O bonds with a lower energy barrier（0.47 e V）.Moreover,hydrogen undergoes dissociation on adjacent Nisites,followed by the hydrogenation of intermediates（C₈H₇NO*and C₈H₇NOH*）on the Ru^δ–sites.This host-dopant synergistic effect in Ru NiSAA catalyst results in outstanding activity and selectivity toward nitroarenes hydrogenation,which can be extended to other rare precious metal catalysts used in structure sensitive reactions.3.Ir/NiO single-atom catalyst towards reductive amination of aldehydesNiO-supported Ir single-atom catalysts（1%Ir/NiO）were synthesized by a two-step method including calcination of hydrotalcite-like Ni（OH）₂ precursor followed by impregnation treatment,whose structure was effectively controlled by finely adjusting the prereduction temperature（1%Ir/NiO-x,x=reduction temperature）.Among them,the 1%Ir/NiO-180 catalyst displays a conversion of100%and furfurylamine selectivity of 99.2%in furfural reductive amination reaction,which is significantly superior than 1%Ir/NiO-160（selectivity:8.4%）,1%Ir/NiO-200（selectivity:85.1%）,and other heterogeneous catalysts ever reported.Moreover,the catalyst exhibits good universality for reductive amination reactions with satisfactory recyclability.HAADF-STEM and DRIFTS of CO adsorption give direct evidences that the atomically-dispersed Ir species are obtained over NiO in 1%Ir/NiO-160 and 1%Ir/NiO-180 samples.By means of XPS,XAFS and in situ FT-IR studies,we demonstrated that the reduction treatment results in the decrease of the Ir-O coordination number and the increase of the Ir-Nicoordination number,which give rise to an enhanced electron density of Ir single atoms,and the formation of a specific Ir-Niinterfacial site in 1%Ir/NiO-180 catalyst.This effectively promotes the adsorption and further reductive amination of reaction intermediate（N-furfurylidenefurfurylamine）,accounting for the production of furfurylamine with a rather high selectivity.This work would pave an avenue for the design and structure optimization of efficient single-atom catalysts.